Image display systems based on spatial light modulators (SLMs) are an alternative to image display systems based on cathode ray tubes (CRTs). SLM systems provide high resolution without the bulk of CRT systems.
Digital micro-mirror devices (DMDs) are one type of SLM, and may be used for either direct view or for projection displays. A DMD has an array of hundreds or thousands of tiny tilting mirrors, each of which provides light for one pixel of an image. To permit the mirrors to tilt, each mirror is attached to one or more hinges mounted on support posts, and spaced by means of a fluidic (air or liquid) gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt. For display applications, image data is loaded to memory cells of the DMD and in accordance with this data, mirrors are tilted so as to either reflect light to, or deflect light from, the entrance pupil of a projection lens and then to an image plane.
One approach to providing color images in an SLM display system is referred to as "sequential color". All pixels of a frame of the image are sequentially addressed with different colors. For example, each pixel might have a red, a green, and a blue value. Then, during each frame period, the pixels of that frame are addressed with their red, green, then blue data, alternatingly. A color wheel having segments of these same colors is synchronized with the data so that as the data for each color is displayed by the SLM, the light incident on the SLM is filtered by the color wheel. For standard display rates, the eye perceives the image as having the proper color.
If the data being displayed and the color wheel are not property synchronized, visual artifacts will occur. One approach to synchronization is to generate an index signal that indicates the location of a designated point on the color wheel. Each time the color wheel completes one rotation, a new index signal is generated. The index signal can then be used to synchronize the color wheel to each new frame of data, such as by comparing it to a reference signal. An example of a reference signal is the vertical sync signal that accompanies a broadcast television signal. The reference signal and the index signal are delivered to a color wheel controller, which maintains the correct color wheel phase relationship with the source signal, as well as the correct rotation speed.
The use of an index signal to control the color wheel motor is described in U.S. Pat. No. 5,365,283, entitled "Color Phase Control for Projection Display Using Spatial Light Modulator", assigned to Texas Instruments Incorporated.
To generate the index signal, some existing systems use a reflective marker on the color wheel that reflects light to a photodetector. Others also use a marker on the color wheel, but one that is magnetic so that it can be detected with a hall sensor. These systems require separate sensor circuitry, special alignment procedures during manufacture, and proper maintenance of the alignment during continued use.